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A930Rocket's Avatar
 
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Leakdown numbers...

So as part of a PPI, a leakdown and compression test were done on my car with 1500-2000 miles on the rebuild. Everything new. See the attached results. Cyl numbers 3 and 5 had issues with the exhaust valves. What's the consensus on them and remedy?

Carbon buildup? Run injector cleaner?

Tight valves? Valve adjustment?

Ideas and thoughts?

Thanks
Jim



Cyl Com ******* Leakdown %
1- 127.5 *******4%
2- 130.0 *******5%
3- 125.0 *******12% Exhaust valve
4- 130.0 *******8%
5- 122.5 *******22% Exhaust valve
6- 132.5 *******5%


Last edited by A930Rocket; 01-13-2011 at 03:51 PM..
Old 01-12-2011, 07:27 PM
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Jim, i guess it could be poor valve adjustment.... It's got to be worth running cleaner through there and get them adjusted.
But if those are track miles, remember a full 24hr race at Le mans is only around 2500 miles, so you could be due a top end rebuild again anyway?

What type of exhaust valve guides did you use on the rebuild?
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Old 01-13-2011, 12:48 AM
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It's only about 200-250 track miles on the rebuild (1 DE and the TurboPalooza track day at CMP 1-1/2 years ago?).

I don't recall what valves or valve guides were installed, but all were new.

I'll adjust the valve clearances, as they've not been checked since the rebuild.
Old 01-13-2011, 01:16 AM
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Hope it's a simple fix for you mate
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Old 01-13-2011, 03:17 AM
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Yeah, me too. I'm kind of surprised about the results.
Old 01-13-2011, 07:52 AM
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Jim,

When you said that cylinders 3 and 5 had issues with exhaust, do you mean that during the leakdown test in those cylinders, you heard air escaping through the exhaust system?

If so, it's possible, I suppose, that those valves could be adjusted "too tight," but that seems somewhat unlikely. They'd have to be so far off that they weren't closing completely, which is pretty extreme. But if that were the case, then they could be damaged from having run in that condition -- that's one way that the edges of valves get burned. Let's hope that isn't what happened.

Do you have any leakdown or compression numbers from right after your rebuild as a baseline to compare with these latest numbers?

What I have seen on these engines is that dirt can get between the valve and seat surface that may prevent it from sealing.

What I'd do is pull the valve covers, check the valve adjustment, and then re-do the leakdown. Do you have the equipment to do it yourself? The tester only costs $100 or so, but you need a compressed air source. If you still have low numbers on those cylinders, then try this: with the leakdown gauge attached and the cylinder at TDC, take a block of wood and a hammer and use it to tap on the valve stem (by putting the wood on the rocker arm adjusting foot). As you tap, you'll see the leakdown needle drop and then rise again. If it consistently rises to the same number each time, then there's probably no dirt (carbon or whatever) under in the valve seat. That would mean that either there truly is a problem at that valve, or maybe, there's a problem somewhere else that's unaffected by your test. If the number rises or changes as you tap, then that would be an indication that you're moving something around in the valve seat (like dirt). You might even "fix" it with this technique! I've seen engines that have sat for a long time that have shown very low leakdown numbers at the valve seat. After tapping the valve in this way, the leakdown numbers came right up to spec, which indicated there was nothing fundamentally wrong with the engine

I think leakdown testing can be pretty flaky, so at a minimum I'd certainly adjust the valves, run the engine, and do the test again, to confirm that there's an issue.

Good luck.

Rob
Old 01-13-2011, 09:44 AM
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Sounds like stuck vavles to me...possibly run some cleaner through the tank, run it hard, and then do another leak down?
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Old 01-13-2011, 10:04 AM
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Thanks guys. Good idea on tapping the valve. And yes it's the exhaust valves on 3 and 5. I just picked up a can of BG fuel cleaner, in addition to the Techron cleaner. I'll check the valves, run the cleaner and try a leakdown test. I have a small compressor and I'm not sure if it up to the job and if not I'll take it back or to another shop. After that, I'll have to see what the options are...

Thinking back, I may only have 1000 miles on the engine since the rebuild with most up front and short hops to the store a few times since.
Old 01-13-2011, 10:49 AM
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I too like Rob930's valve tapping technique, definitely give it a try.

Are you getting any exhaust smoke on startup after sitting cold a few days?
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Old 01-13-2011, 12:19 PM
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Some yes, but the car will sit for weeks at a time. I consider that normal.

Quote:
Originally Posted by William930t View Post
I too like Rob930's valve tapping technique, definitely give it a try.

Are you getting any exhaust smoke on startup after sitting cold a few days?
Old 01-13-2011, 12:33 PM
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I had the same problem, went by the book on a rebuild, everything new but then found out after some miles about the "little things", like the fact that valve guides are supplied to shops already sized. What this means is that when they are pressed in by a shop, they bell up in the middle of the guide and have to be reamed to get the valve in (recall that the guides were already sized). So what you end up with is sloppy outer tolerances which eventually beat themselves out, sooner if not later, causing all kinds of problems.
Old 01-13-2011, 03:09 PM
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EBS did all the machine work, so I think I'm OK there, but it's good info to know.
Old 01-13-2011, 03:48 PM
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I had my heads done by a well known shop, too...equal to EBS, if not better. When i took the motor down again, i considered the guides fine. A few of my cylinders showed some wetness in the exhaust ports and there was smoking issues, leak down issues. Into the build -- or should I say the rebuild #2 -- I decide to get the new guides checked out. I was totally amazed to see them totally shot! They were ruined. I could not believe it. I saw this with my own eyes. I did not place blame but only learned from my adventure. This is a real issue that needs to be understood. It does make perfect sense. The guide only has a small middle portion that has the correct diameter when reamed, the rest of the guild becomes oversized, so the valve stems slap around and beat themselves out. The solution was to get undersized custom guides pressed in and then the the entire guide is reamed to spec. Cured! no problems, smooth as silk and runs better than ever.
Old 01-13-2011, 04:23 PM
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Just for perspective...

When I had my 930 motor out I did a leakdown on it. 3-5% or so on 5 cylinders, 40-50% on another. I was about to cry...metaphorically speaking of course .

The valve looked fine, but had some carbon on it. Shot some carb cleaner on it, and it was hissing/sealing poorly.

Adjusted the valve, no difference...

Took a paint gun cleaning bristle, and ran it around the valve base for about 2-3 minutes. Nothing seemed to pop up, but there was some miniscule carbon. Poured some valve cleaner onto the closed valve, let it sit for several hours... Redid the leak down, and it was perfect. Shot some more carb cleaner on the valve while closed and let it sit - no bubbling.

Moral of the story : I was shocked how tiny the imperfection in the way the valve seats screws up the leakdown...
Old 01-14-2011, 06:45 AM
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But then the story of mine was similar figures to Jim, heads off and ALL of the exhaust valve guides were 'totally' shot, and only one cylinder had bad figures! I guess you don't know until you get in there!
Hopefully with such low mileage it will be the simple fix, think my heads had done 40ishK miles, and a good 20k of those at a reasonably high power level.
As i said earlier Jim, got my fingers crossed for you that it's something and nothing! Keep us updated
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Old 01-14-2011, 06:56 AM
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The car is up in the air and I'm about to start work on it. Be back in a few hours...
Old 01-14-2011, 09:53 AM
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Quote:
Originally Posted by zcoker View Post
This is a real issue that needs to be understood. It does make perfect sense. The guide only has a small middle portion that has the correct diameter when reamed, the rest of the guild becomes oversized, so the valve stems slap around and beat themselves out. The solution was to get undersized custom guides pressed in and then the the entire guide is reamed to spec. Cured! no problems, smooth as silk and runs better than ever.
Reading this thread made me nervous...with just 4k miles on my DIY rebuild.

I spoke with my machinist who confirmed this is a problem with the factory valve guides. I am happy to say that my heads were done with custom bronze-manganese guides that are reamed the full length to fit.

I sure hope it turns out to be nothing serious for you Jim (praying to the 930 gods)
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Old 01-14-2011, 01:40 PM
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Hey Jim,

I'll have to second what Rob mentions below....tapping on the valve is well worth the effort, especially if it helps that valve take a set and give you a proper reading.

Having done this with Rob in the past...seeing is believing!

Quote:
Originally Posted by Rob 930 View Post
Jim,

When you said that cylinders 3 and 5 had issues with exhaust, do you mean that during the leakdown test in those cylinders, you heard air escaping through the exhaust system?

If so, it's possible, I suppose, that those valves could be adjusted "too tight," but that seems somewhat unlikely. They'd have to be so far off that they weren't closing completely, which is pretty extreme. But if that were the case, then they could be damaged from having run in that condition -- that's one way that the edges of valves get burned. Let's hope that isn't what happened.

Do you have any leakdown or compression numbers from right after your rebuild as a baseline to compare with these latest numbers?

What I have seen on these engines is that dirt can get between the valve and seat surface that may prevent it from sealing.

What I'd do is pull the valve covers, check the valve adjustment, and then re-do the leakdown. Do you have the equipment to do it yourself? The tester only costs $100 or so, but you need a compressed air source. If you still have low numbers on those cylinders, then try this: with the leakdown gauge attached and the cylinder at TDC, take a block of wood and a hammer and use it to tap on the valve stem (by putting the wood on the rocker arm adjusting foot). As you tap, you'll see the leakdown needle drop and then rise again. If it consistently rises to the same number each time, then there's probably no dirt (carbon or whatever) under in the valve seat. That would mean that either there truly is a problem at that valve, or maybe, there's a problem somewhere else that's unaffected by your test. If the number rises or changes as you tap, then that would be an indication that you're moving something around in the valve seat (like dirt). You might even "fix" it with this technique! I've seen engines that have sat for a long time that have shown very low leakdown numbers at the valve seat. After tapping the valve in this way, the leakdown numbers came right up to spec, which indicated there was nothing fundamentally wrong with the engine

I think leakdown testing can be pretty flaky, so at a minimum I'd certainly adjust the valves, run the engine, and do the test again, to confirm that there's an issue.

Good luck.

Rob
Old 01-14-2011, 05:46 PM
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I checked the valve clearance and it was good. Then I took a brass drift and brass hammer to tap on the valve. I wasn't sure how hard to tap on it, but tapped on it 10-20 times. Checking with a leakdown after the fact showed it was still coming out the exhaust valve. Didn't think to air it up and tap at the same time. I'll try that next time.

How hard do I tap it?

Plan on taking it for a good drive tomorrow with the injector cleaner, then back home to check it again. Maybe rap on the valve again and retest with air.
Old 01-14-2011, 06:16 PM
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Jim,

Did you get a chance to check the exhaust valves after our drive on Saturday?

There might be an outside chance that the test guage was out of calibration on the last two readings ... 1-6-2-4-3-5

Might be a reach, but if the test was done by firing order and not by bank then it wouldn't hurt to retest.

Also wondering if you could hear leaking on the cylinders that had good numbers. Just so that you can compare to those that are in question.

Good luck. Like everyone else, I'm hoping for the best. You have a beautiful car.

Craig

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